Changing the weekStart value (even locally) breaks the format function globally

[jordwest]

Describe the bug
Changing the weekStart value breaks the format function, when changing locale locally or globally.

Minimal reproduction steps:

// This works:
dayjs().format('DD MM YYYY')
// > "07 12 2020"

// Update locale *locally* or globally
dayjs().locale('en', { weekStart: 1 }).valueOf();
// > 1607303261223

// Now this breaks:
dayjs().format('DD MM YYYY')
// > Uncaught TypeError: i is undefined

Expected behavior

I would expect that changing the locale locally to have no effect on other dayjs instances. This suggests there's some global state leaking out of the instance locale function.

Information

• Day.js Version: v1.9.7
• OS: Mac OS Catalina 10.15.4
• Browser: Confirmed in Firefox 83 and Chrome 86
• Time zone: GMT+11:00 AEDT (Australia/Sydney)

Likely related to #765 (however updateLocale doesn't work for me as I'm changing locale locally, not globally)

[iamkun]

If you want to change the locale locally, you have to pass in the whole locale object.

import en from 'dayjs/locale/en'

const newEn = copy(en)
newEn.weekStart =  1

dayjs().locale(newEn)